ISOLATION, EXTINCTION AND MIGRATION WITHIN LATE PLIOCENE POPULATIONS OF THE PLANKTONIC FORAMINIFERAL LINEAGE GLOBOROTALIA (GLOBOCONELLA) INTHE NORTH-ATLANTIC

Quantitative records of Globorotalia puncticulata and Globorotalia inf lata, the last two members of the Globorotalia (Globoconella) lineage, obtained from North Atlantic sediments collected at DSDP Site 552, OD P Site 659 and ODP Site 665, are used to examine fluctuations in the b iogeographic distribution of these species in the Late Pliocene betwee n 3 and 2 Ma. Abundance data indicate that prior to the expansion of N orthern Hemisphere glaciation at about 2.5 Ma, Gr. puncticulata was an important component of the planktonic foraminiferal fauna and had a g eographic distribution ranging from 2 degrees N to at least 56 degrees N in the North Atlantic. A previously undescribed 6 chambered variant of Gr. puncticulata is found at both Sites 659 and 665. The stratigra phic distribution of this morphotype is restricted, first occurring at 2.9 Ma and then disappearing when glacial intensity increased at 2.75 Ma (isotope stage 110). Similar declines in Gr. puncticulata abundanc es occurred during glacial isotope stages 102, 100, and 98 immediately prior to the extinction of Gr. puncticulata during glacial isotope st age 96. It appears that this extinction event was latitudinally diachr onous within the North Atlantic, occurring earliest in the north at Si te 552 (2.453 Ma), then at Site 659 (2.443 Ma) and later still in the Site 665 equatorial record (2.438 Ma). At Site 665 the first record of Gr. inflata occurs during glacial isotope stage 94 (2.416 Ma), shortl y after the extinction of Gr. puncticulata. In the mid latitude North Atlantic there was a 340,000 year period following the disappearance o f Gr. puncticulata when the Globoconella lineage was absent (the Gr, i nflata gap), The Gr. inflata population found in the equatorial Atlant ic must therefore have been introduced from the South Atlantic, probab ly by the South Equatorial Current. Faunal records from Sites 552 and 659 show that it was not until glacial isotope stage 78 (2.10 Ma) that Gr. inflata became widely established in the North Atlantic. Prior to this large-scale migration event, there were two limited colonisation events during glacial isotope stages 86 and 82 when Gr. inflata popul ations reached as far as Site 659 in the eastern North Atlantic. These incursions are believed to be reflect the entrainment of Gr. inflata within South Atlantic Central Water and the northward subsurface trans port of individuals to the coastal upwelling zone off northwest Africa . It seems likely that the same mechanism was responsible for the re-e stablishment of the Globoconella lineage in the North Atlantic at 2.10 Ma, but in this instance additional factors, such as enhanced glacial circulation patterns and ecological changes within planktonic foramin iferal faunas, resulted in the successful expansion of Gr. inflata acr oss the North Atlantic and the Mediterranean. (C) 1998 Elsevier Scienc e B,V. All rights reserved.